Lost 4-5 mpg overnight

[FlatTwin]

Gents,Have been slowly upgrading my 2500 Cummins and getting it the way it should be. With everyone's assistance it now has a full set of gauges and a Raptor LP and drawstraw. Using a Edge/Attitude but its pretty much there just for the gauges. The wire to the VP44 is not even hooked up so running at level 1. Had the truck for a year or so and know it pretty well. Was getting 23-24 mpg on the overhead gauge in town, and 26-27 on the highway. No towing, running 60-65 or so, usually not faster. Did some work over in SoCal recently so filled up on the Arizona side of the border ($2.72!) and headed home. Did 670 miles before filling up again and it took 27 gallons. Overhead display showed 23.5 mixed city and highway driving. Actual MPG reading using a calculator was better, showing 24.5 or so. Was very happy with that. This is the mileage I have gotten for the last year I have had the truck.Took the truck to get an oil change at a place I have used for years a month ago. Very good guys. Changed the oil and rotated the tires, nothing else done to the truck. Mileage has now dropped overnight to 22.5 for either highway or city driving. Checked the two month old air filter and it looks great, and the "bypass" gauge on the filter housing confirms that. Air pressure in the tires is fine, running 14-17 psi fuel pressure. All other readings appear to be exactly the same as before. Fuel filter is 3 months old at most.Anyone have any suggestions on what this could be? Just took the truck back to the shop and they found nothing. Front tire pressure was a bit low but thats it.Looking everything over closely the rear brakes were pretty far gone so now have new pads in there but that does not appear to have caused anything with the fuel mileage. Cannot figure out anything else unless my VP44 is giving up the ghost but do not feel that something like this would happen overnight like this. Thx,Joe (FT)

[dorkweed]

Moved to 2nd gen powertrain forum.

[cajflynn]

Are you losing power also? It could be your vp going bad.

[FlatTwin]

No power loss that I can feel just yet. Was thinking that it might be the VP but hoping not.How substancial of a power loss is it? Just a bit or a lot? Thx,Joe

[gassernomore]

Are you saying you lost 4-5 mpg's off the overhead? The overhead is very inaccurate. Mine has fluctuated all the way upto 99 mpg's. Stay with hand calculations or get a ScanGaugeII for mpg accuracy. You can check for codes as well. Key on, off, on, off, on (not starting it). Look at the odometer, write down any codes such as P...., Wait for it to say "done" twice. Repost the codes. Good Luck.

[FlatTwin]

Thanks for the ideas.I have a scan-gauge on the truck (should be listed in my signature) and its telling me the same thing that the overhead does. Am not showing any codes at this time, and never had any pop up. That said, my overhead has been accurate. Only owned the truck for a year now but its been dead nuts on, if not just a bit low on the actual mileage. Every time I have checked it the overhead has been correct.Thx,Joe

[Freelancer88]

You said the rear brakes were far gone, but have you checked the front brakes? The only thing I could think of is your brakes are dragging, if not fronts, maybe the e-brake??

[FlatTwin]

E-Brake seems fine and the rear rotors showed no sign of overheating, so do not believe its that.We pulled the front wheels and the pads are like new there. Again no sign of overheating on the front rotors either.

[dorkweed]

How cold has it been by you??? My winter mileage drops every year due to winter blended fuel and cold weather. The cold weather (below freezing and down to below 0) require more fuel to warm up, more fuel to pump that cold oil around the engine, more fuel to overcome the thick oil in the diffs and tranny and transfer case. Cold air is also denser which requires more energy to push through.You could've gotten a bad tank of fuel or fuel with a higher than normal bio % in it. That's if bio is sold around your parts.

[FlatTwin]

Winter in Phoenix? Might get down to 35 degrees two or three times a winter, otherwise tonight its going to be a low of 45.Fuel is pump fuel from the Flying J. No bio-diesel in the truck that I know of.

[gassernomore]

With having a ScanGaugeII I would go ahead and check for codes as well. Codes don't always light up the CEL.

[Mopar1973Man]

Hmmm... How about a brake caliper binding up? I've seen this drop MPG over night but not given the smell of hot brakes... When was the last time you full disassembled the brake calipers and clean and lube the parts (slide pins and such)?